The troops were backed by pro-regime militia and fighters from Lebanese militant group Hezbollah in their assault on rebel groups including the Al-Qaeda-affiliated Islamic State of Iraq and the Levant.

"Regime troops have advanced inside the base and now control large parts of it, and rebel groups and the Islamic State are bringing in reinforcements," said the Observatory.

Initial reports suggested both sides had suffered casualties in the fighting, and the Observatory added that towns near the base had come under heavy bombardment, along with neighbourhoods of Aleppo city.

Rebel forces seized control of Base 80 in heavy fighting in February.

The base's garrison was responsible for securing Aleppo international airport and the adjacent Nayrab military airfield, which both remain under regime control.

Elsewhere, the Observatory reported ongoing fighting around a key weapons depot near the town of Mahin in central Homs province.

Rebel groups, including Al-Qaeda-affiliated battalions, seized control of part of the depot earlier this week, and the Observatory said the army and opposition were bringing in reinforcements.

More than 120,000 people have been killed in the conflict in Syria since March 2011, according to the Observatory. Millions more have been internally displaced or fled abroad.